A demand exists for cellulose fiber containing nonwoven materials that are colored, have textile aesthetics and performance, and remain fast under harsh chemical and abrasive use. It is highly desirable for such nonwoven materials to be laundrable and durable. It is also desirable for such substrates to be lightfast.
These nonwoven materials can be used to replace traditional textiles in applications including, but not limited to, wipers, wearing apparel, equipment protection, and bedding. Such products are used in a wide range of industries including: manufacturing, medical, printing, spray paint, garment and food services.
Insoluble colorant pigments are used to color cellulose fiber containing nonwoven materials. These pigments are generally inorganic or contain a synthetic organic base. A fixing agent is typically used to improve fastness because these colorant pigments are insoluble in the application medium and do not readily migrate into cellulose fibers or fix onto them. Useful fixing agents include alum, caseins, starches, acrylics, rosin sizes, polyvinyl alcohols, and cationic colorant fixatives. Generally speaking, these fixatives only modestly improve durability.
Soft polymeric adhesive binders or resins are also used as fixing agents. They improve durability by encapsulating and binding the insoluble pigment to fiber surfaces. Binders and resins have limited use because they are a surface treatment and generally have only moderate fastness. Deeper shades of color require excess pigment and binder or resin that tend to rub off or crock. Moreover, high levels of pigment act as fillers and can physically weaken a sheet. Binders or resins also stiffen nonwoven materials and impair textile-like aesthetics while often negatively impacting liquid distribution and absorbency properties.
Binders and resins are often soluble in many common volatile and semi-volatile commercial and industrial liquids and solvents and could leach from the nonwoven material leaving undesirable residues and streaks. When used on hot surfaces or at high temperature, binder or resin on colored nonwoven materials may migrate, soften, degrade, alter the nonwoven material properties and/or leave residues. Another disadvantage of binder and resin coloring systems is that they are often added to dried sheets using size presses, saturation techniques or printing operations and then again dried. Many binders are also applied as a secondary process off-line to the basesheet production which also increases costs.
Dye colorants are also used to color cellulose fibers and cellulose fiber containing nonwoven materials. Dyestuffs, dye colorants, or dyes are generally categorized into numerous classes according to application. These categories include: basic, acid, direct (including cationic directs), mordant, azoic, disperse, reactive, sulfur and vat dyes. These dyes have a wide range of cost, dyeing properties and fastness. In addition, the method of applying such dyes varies widely from simple introduction to suspended stocks and webs to multistage chemical processes.
Dyes are physically or chemically bonded to fiber to provide durable color. They are bonded typically by one or more forces including physical entrapment, hydrogen bonding, van der Waals forces, coordinately bonded, ionic forces or covalent bonds. Generally speaking, dyes are usually fast or permanent in only some aspects or under certain conditions.
It is desirable for dye colorants to be resistant to light and water. It is also desirable for a dye colorant to withstand other influences encountered in commercial and industrial applications of cellulose fiber containing nonwoven materials. These include, but are not limited to, bleaches and detergents used during laundering and soaking for stain removal; cleaners including acids such as vinegar and bases; and a large list of industrial chemicals including oils, cutting oils, and solvents having a wide range of dipole moments such as: acetone, methylene chloride, 1,1,1 trichloroethane and various alcohols, ketones, benzene, naphthalene and mineral spirits.
Generally speaking, basic dyes have poor light fastness and are susceptible to uneven coloring of cellulose fibers (e.g., paper fibers). Acid dyes are readily susceptible to water bleeding because of their low affinity to cellulose fibers. Direct or substantive dyes will color cellulose fibers without the use of dyeing assistants or mordants. However, they tend to lack the overall chemical fastness needed even with the use of mordanting, cationic fixing agents, formaldehydes or coupling compounds. Direct dyes lack overall fastness since the forces binding them are easily broken.
Generally speaking, mordant dyes have no affinity for cellulose fibers and require use of a metallic oxide treatment for good fastness properties. Azoic dyes require coupling of two dye components onto the fiber but lack overall chemical fast requirements and are normally limited to only a few cellulosic applications. Disperse dyes are typically used to color hydrophobic fibers and are fine-size organic compounds with limited solubility and crock resistance.
Reactive dyes can be described as acid, basic or mordant dye with an attached reactive group that is capable of covalent bonding to a cellulose fiber.
Good fastness is typically obtained by converting soluble compounds into relatively insoluble compounds within the fiber. Sulfur and vat dyes are insoluble and therefore must be chemically modified before coloring fiber. With these dyes, the insoluble dye is first reduced to the soluble leuco compound and after integration into fiber, oxidized back to the insoluble form using typically sodium sulfide for sulfur dyes and sodium perborate for vat dyes.
Cellulose fibers may be dyed utilizing a variety of methods ranging from dyeing individual fibers to consolidated webs and by dyeing at points within the nonwoven web construction process. Exemplary methods include beater or stock coloring within the slush or slurry to dyeing webs by padding, jig dipping, dyebaths, squeezing, extraction operations, foam curtain dyeing and printing. Many of these methods are off-line textile finishing processes.
Specialized pad-batch, pad-thermofix, and pad-steam methods and modified versions for continuous operations with numerous steps have also been developed for reactive dyes by padding the web with dye solution. The web is then either stored for extended reaction times in a vapor tight enclosure or steam heated, further padded, and afterwards the web is washed of spent chemical.
Low speed continuous pad-jig methods and pad-steam methods are often employed for permanent dyeing of webs with vat dyes. Suitable reaction times have been achieved especially at elevated temperatures. After chemical dyeing using reactive and vat dyes, a washing step(s) is added to remove unreacted exhausted chemicals since the reaction is not 100% complete. More permanent colorants generally require several chemical process steps and extended reaction times.
While reactive dyes, vat dyes and sulfur dyes appear desirable for use with cellulose fibers, application of these dyes requires more than one process step and is often hampered by slow line speeds needed to achieve adequate reaction times.
Accordingly a need exists for a simple process for applying reactive dyes, vat dyes and sulfur dyes to cellulose fibers and to cellulose fiber containing nonwoven materials to produce durable coloration. This need extends to a continuous or one-step process for applying such dyes to the described substrates so they are colorfast. This need also extends to a process for applying such dyes that is suitable for high-speed manufacturing processes. There is also a need for colorfast cellulose fibers, nonwoven materials containing colorfast cellulose fibers, and colorfast nonwoven materials that include cellulose fibers that are prepared in a simple, one-step process.